1. Field of the Invention
This invention relates to a sol-gel process for fabricating glass and ceramic monoliths with selected properties (e.g., chemical purity, homogeneity) compatible with a variety of applications, including high-performance optics.
2. Description of the Related Art
High-performance oxide-based materials are increasingly in demand for use in a variety of applications. For example, silica glass has the optical transmittance, mechanical hardness, chemical durability, thermal stability, low thermal expansion, and high laser damage threshold which make it an optimal material for applications such as optoelectronic laser diodes, fiber optic telecommunications, medical laser delivery systems, and military optical sensors. There is significant pressure on material manufacturers to find fabrication techniques which can satisfy the increasingly stringent performance requirements for silica glass and other oxide-based materials.
Numerous techniques are currently in use for the fabrication of glasses or ceramics. For example, silica glasses have traditionally been manufactured by melting natural quartz or synthetic silica in crucibles at high temperatures (typically about 1700° C.–2000° C.). However, the resultant materials have limited utility for various optical applications, primarily due to structural inhomogeneities as well as impurity concentrations (e.g., from intrinsic impurities in the raw materials, incomplete chemical reactions of components, and contamination by the crucible). Such high-temperature processes are also unsuitable for manufacturing products with certain compositions, tailored dopant or additive gradients, organic or high vapor pressure additives, or additives in their metallic or partially reduced states.
Another more recent technique for manufacturing silica glasses has been chemical vapor deposition (CVD), in which silicon-containing chemical vapors are combined with oxygen under high temperature conditions to deposit silica onto a substrate. However, the resultant materials are relatively expensive due to low material collection efficiencies, slow processing rates, and complex processing and pollution control equipment. Furthermore, CVD processes lack the versatility for fabricating more compositionally complex glasses.
Sol-gel technology has been used to fabricate products which satisfy some or all of the desired performance requirements without the difficulties or limitations found in more conventional fabrication techniques. A typical sol-gel silica process involves the transition of a liquid colloidal solution “sol” phase into a solid porous “gel” phase, followed by drying and sintering the resulting gel monolith at elevated temperatures to form silica glass. One method of preparing a silica porous gel monolith is to pour into a mold a solution of silica-forming compounds (e.g., silicon alkoxides), solvents, and catalysts, which then undergoes hydrolysis and polymerization, resulting in a wet porous gel monolith or matrix. After drying the wet gel monolith in a controlled environment to remove the fluid from the pores, the dry gel monolith is densified into a solid glass-phase monolith.
Sol-gel technology can yield products with the desired chemical purity, homogeneity, and flexibility in compositions, dopants, and dopant profiles. However, the potential for sol-gel processes for fabricating large monoliths has been limited by various problems. Large gel monoliths can take a long time to dry, thereby limiting the product throughput. But even more importantly, shrinkage of the gel monolith during the drying process often results in cracking, especially in larger gel monoliths.
As outlined by Pope, et al. in U.S. Pat. No. 5,023,208 and Wang, et al. in U.S. Pat. No. 5,264,197, both of which are incorporated by reference herein, this resultant cracking of gel monoliths during the drying step of the fabrication process is believed to result from stresses due to capillary forces in the gel pores. Numerous techniques for reducing this cracking have been proposed, and many of these efforts have focused on increasing the pore sizes of the gel monolith to reduce the capillary stresses generated during drying. Pope, et al. discloses subjecting the gel to a hydrothermal aging treatment which causes silica particles to migrate and fill small pores in the porous gel matrix, thereby increasing the average pore size. Wang, et al. discloses adjusting the relative concentrations of an alcohol diluent and/or one or more catalysts such as HCl or HF, which has the effect of increasing the average pore radius of the resulting dry gel. HF catalyzed gels generally have larger pore sizes than gels catalyzed by other catalysts such as HCl, HNO3, H2SO4, or oxalic acid.